A typical wireless communication system is composed of two or more transmitter/receiver nodes adapted to communicate with each other. To accomplish a message exchange or data communications, between nodes, each node is adapted to selectively switch between transmit and receive modes by local node control. Communication systems use time, frequency and code division multiplexing to ensure only a single transmitter is active at any given instant in time, for a given set of times, frequencies and codes, within the geographical region defined by the wireless signal propagation characteristics of the communicating nodes
Two or more communicating nodes within a geographical region comprise a wireless communications network. All nodes in the network are adapted to synchronize in time, frequency and code, in a manner that enables successful communication. The precision, or resolution, of the synchronization determines, in part, the overall communications network performance and efficiency. In particular, the need for time synchronization among nodes is typically required to provide a means for determining when each node should transmit or receive. Each node typically includes a local clock, which is synchronized with the local clocks in other nodes according to a scheme employed by the particular wireless network. The synchronization process typically includes exchanging a sequence of information known to all nodes, called a preamble, which is transmitted by one node and received by one or more other nodes. Each receiving node adjusts the local clock to match the transmitted preamble sequence, and achieve time synchronization.
As a function of the resolution available to synchronize the clocks, some ambiguity may exist between two or more clocks, potentially leading to the case where two or more nodes are transmitting simultaneously, an undesirable situation in most wireless networks. A means of avoiding this situation is to provide a “guard band” time after each node completes a transmission, prior to a different node starting to transmit. The duration of the guard band time is sufficiently long to ensure any clock ambiguity is resolved, thereby avoiding the situation with multiple simultaneously transmitting nodes.
The clock in each node operates with a specified stability, independently from the clocks in all other nodes, and as a result may drift out of synchronization with the clocks in one or more other nodes. The required accuracy, or tolerance, of time synchronization among the node clocks is determined by the type of communication signals exchanged between nodes. It is therefore necessary to periodically perform a clock synchronization activity involving all communicating nodes. The time interval between such synchronization activities is referred to as a communications frame.
The nodes in many wireless communications networks are powered by energy sources exceeding the requirements of the node. The duration of the communications frame is typically limited only by the clock drift in the node, and may include generous guard band periods for ensuring only a single node is transmitting at a time. Nodes in other wireless communications networks may be powered by limited capacity energy sources, such as batteries. In wireless networks with such energy-constrained nodes, the communications frame must be carefully designed to be energy-efficient, with minimal duration guard bands, so the node's maximum operational time from the energy source can be obtained.
During the communications frame time period, the nodes in the wireless communications network exchange information, with one node transmitting at a time, and one or more other nodes receiving the information. The information is typically encoded in a time-varying signal created by the transmitting node, and decoding process relies on the transmitting and receiving nodes to be time-synchronized, thus all information exchange occurs during the communications frame, following the preamble, and any applicable guard band periods.
Wireless communications networks may perform sensing and control applications by including in each node one or more sensors or actuators. When dispersed throughout a region, multiple nodes in such a network may function in a collaborative manner to provide a sensing or actuating mechanism with a scope greater than possible with a single node. For example, a passive sensing application may include multiple nodes, each creating a time-stamp of when a particular acoustic signal was detected. Based on the different times-of-arrival (ToA) of the acoustic signal at each node, it is possible to determine the position of the sound source relative to the receiving nodes. Since it is possible the ToA period at one or more nodes may exceed the communications frame duration, it would be necessary to perform the clock synchronization activity during the ToA measurement period of the sensed acoustic signal, in order to ensure the nodes remain in time-synchronization for the purpose of creating a time-stamp when the acoustic signal is sensed.
For energy-constrained wireless sensor nodes, the need to maintain clock synchronization suitable for information exchange during a sensing or actuation activity is sub-optimal for collaborating nodes, since new information to exchange is available only after the collaboration has completed. However, such collaboration may require the collaborating nodes to maintain time-synchronization, though with a coarser resolution than for information exchange.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the communication industries for a method to provide collaboration among two or more transmitter/receiver nodes that reduces multiple re-synchronization preambles and minimizes energy consumption at each node and utilizes the residual clock synchronization period remaining after data communication is completed in the current communications frame.